The present invention relates to an information retrieval system and, more particularly, to an information retrieval system adapted to perform an information retrieval by inputting an inquiry sentence in natural language, executing a semantic analysis of the natural language inquiry sentence by a semantic analysis unit, and producing retrieval conditions from a result of the semantic analysis.
As data base technology and artificial intelligence technology develop, there have been growing demands for an information retrieval system which may be used by users inexperienced in computers, as well as skilled operators. A system has been developed as one example in compliance with such demands, which may make an inquiry in natural language with a computer. Such an information retrieval system is provided with a semantic analysis unit for processing natural language, which unit is designed so as to understand the semantics of an inquiry sentence inputted in a natural language, produce retrieval conditions, and carry out an information retrieval. In order for the semantic analysis unit of the system to understand the semantics of words contained in the inquiry sentence, it is required to collate a word with a dictionary and carry out a semantic analysis. It is to be noted, however, that it is impossible to register in a dictionary all words which might be contained in a great variety of inquiry sentences so that a portion of words are not capable of being collated, such as an undefined word, whereby the system cannot understand the inquiry sentence.
A natural language processing technology including a semantic analysis is the type of technology provided for a machine translation system. In the machine translation system, various techniques are used for processing undefined words for which it cannot carry out a semantic analysis if there is an undefined word. For example, Japanese Laid-Open Patent Application (kokai) No. 60-181,973 discloses a system for recognizing undefined words, which system is designed to investigate a grammatical relationship of an undefined word with a word located before and after the undefined word and recognize a grammatical attribute of the undefined word.
It is noted, however, that conventional undefined word processing technology relates in almost all cases to machine translation systems which are designed to recognize a grammatical attribute, for example, from a word before and after an undefined word, a suffix, a prefix or the like. Such a system for recognizing a grammatical attribute of an undefined word from a grammatical relationship of the undefined word with a word therebefore or thereafter suffers from the disadvantage that, if there is an undefined word that cannot be recognized as a result of investigation of its grammatical relationship, recognition processing of such a grammatical relationship cannot be continued, leading to inability to analyze the semantics of a sentence.
As have been described hereinabove, in a semantic analysis for a machine translation system, on the one hand, a semantic analysis often becomes stalemated and a continuation of the processing becomes very difficult or hardly possible if there is an undefined word that is unrecognizable with respect to its attribute from the grammatical relationship. In a semantic analysis in an information retrieval system, on the other hand, there are many cases in which no continuation of the processing can be blocked in processing of retrieval conditions for undefined words even if there would be an undefined word whose meaning cannot be recognized from its grammatical relationship, when a semantic analysis of an inquiry in natural language is carried out. Even in this case, however, conventional information retrieval systems suffer from the disadvantage that the presence of an undefined word leads to suspension of continuation of the processing with a semantic analysis being disabled as in the case of a semantic analysis in the machine translation system.